1. Field of the Invention
The present invention relates to a reversible thermosensitive recording material, which reversibly changes its color when appropriately heated and then cooled. In addition, the present invention also relates to a method for recording and erasing an image in the reversible thermosensitive recording material.
2. Discussion of the Background
A variety of reversible thermosensitive recording materials have been proposed. For example, they are as follows:
(1) a recording material which includes a combination of gallic acid with phloroglucinol as a color developer (Japanese Laid-Open Patent Publication No. (hereinafter JOP) 60-193691);
(2) a recording material which includes phenolphthalein or Thymolphthalein as a color developer (JOP 61-237684);
(3) a recording material which includes an uniform solid solution of a coloring agent, a color developer and a carboxylic acid ester (JOPs 62-138556, 62-138568 and 62-140881);
(4) a recording material which includes an ascorbic acid derivative as a color developer (JOP 63-173684); and
(5) a recording material which includes a higher aliphatic amine salt of bis (hydroxyphenyl) acetic acid or gallic acid as a color developer (JOPs 2-188293 and 2-188294).
JOP 5-124360 discloses a reversible thermosensitive coloring composition which includes a coloring agent such as a leuco dye and a color developer such as an organic phosphate compound, an aliphatic carboxylic acid compound or a phenolic compound each of which has a long-chain aliphatic hydrocarbon group, and a reversible thermosensitive recording material using the coloring composition. The reversible thermosensitive recording material can stably repeat image formation and erasure by being appropriately heated and cooled, and the image-recorded state and the image-erased state can be stably maintained at room temperature. In addition, JOP 6-210954 discloses a reversible thermosensitive recording material which includes a specific phenolic compound having a long-chain aliphatic hydrocarbon group as a color developer.
As mentioned above, various reversible thermosensitive recording materials capable of repeatedly performing color formation/erasure have been proposed. However, when image formation/erasure is repeatedly performed under practical image recording/erasing conditions, problems such that image qualities of the recorded image deteriorate and/or the recorded image has omissions tend to occur. Therefore a reversible thermosensitive recording material which can fully exhibit the color formation/erasure properties of its coloring composition including a color developer and a leuco dye has not yet been provided.
The reason for the deterioration of the image qualities is considered as follows. When an image is recorded in a recording material, the recording material is heated to a high temperature by a thermal printhead upon application of a mechanical force to the recording material. Therefore, the structure of the recording layer and protective layer tends to change and be damaged when repeatedly used. In order to prevent deterioration of image qualities, the mechanical durability of the recording material should be improved while the mechanical force applied to the recording material is decreased.
Image omissions are produced due to dust adhered to a thermal printhead. Such dust is formed on a thermal printhead, for example, when the thermal printhead touches a recording material on which one or more constituents of the recording material such as a color developer, which migrate from the inside of the recording material to the surface thereof, are deposited; a damaged portion or a peeled portion of the surface of the recording material; or when foreign particles adhered on the surface of the recording material are transferred to the thermal printhead. Therefore the image omission problem can be prevented by preventing adhesion of dust on the thermal printhead used.
Thus, in order that a reversible thermosensitive recording material is practically used in market, the recording material preferably has both a good head-matching property (i.e., a property such that the recording material does not stick to a thermal printhead (i.e., does not cause a feeding problem) and therefore does not receive excessive mechanical force), and a dust cleaning function such that the recording material cleans dust adhered to the thermal printhead used.
In addition, it is needed for a recording material to have a good visibility, i.e., an ability of exhibiting vivid color images. The color tone of recorded images is changed by light scattering on the surface of the recording material. When the color tone of an image deteriorates, contrast between the image with its background decreases, resulting in deterioration of image visibility of the recording material.
In attempting to solve such problems, JOP 08-156410 discloses a recording material in which a protective layer having specific gloss and surface roughness is formed on a recording layer to improve the head-matching property thereof. The recording material has an improved head-matching property but the color tone of recorded images deteriorates due to light scattering on the surface of the recording material. Therefore, the recording material has poor image visibility.
JOP 02-258287 discloses a transparent/opaque type reversible thermosensitive recording material in which a protective layer having a specific surface roughness is formed on a thermosensitive recording layer which records and erases an image by changing its transparency upon application of heat thereto to improve the dust-cleaning property of the recording material. This transparent/opaque type reversible thermosensitive recording material can record and erase an image at a relatively low heat energy. However, a coloring/non-coloring type reversible thermosensitive recording material needs a relatively high heat energy compared to the transparent/opaque type reversible thermosensitive recording material to record an image. Therefore the dust-cleaning property of the coloring/non-coloring type reversible thermosensitive recording material cannot be fully improved by this technique.
Because of these reasons, a need exists for a coloring/non-coloring type reversible thermosensitive recording material which has good image formation/erasure property and can repeatedly record/erase images having good visibility for a long period of time without causing the image-quality deterioration problem and the image omission problem.
Accordingly, an object of the present invention is to provide a coloring/non-coloring type reversible thermosensitive recording material has good image formation/erasure property and can repeatedly record/erase images having good visibility for a long period of time without causing the image-quality deterioration problem and the image omission problem.
Another object of the present invention is to provide a reversible thermal image formation/erasure method by which images having good visibility can be repeatedly formed and erased for a long period of time without causing the image-quality deterioration problem and the image omission problem.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a reversible thermosensitive recording material having a substrate; and a recording layer located overlying the substrate, which includes a crosslinked resin, an electron donating coloring compound and an electron accepting compound, and a protective layer located overlying the recording layer and including a filler and a crosslinked resin, wherein the recording layer in a non-colored state achieves a colored state when heated at a temperature not lower than an image forming temperature and then cooled at a first cooling speed, and the recording layer in the colored state achieves a non-colored state when heated at a temperature lower than the image forming temperature and not lower than an image erasing temperature or when heated at a temperature not lower than the image forming temperature and then cooled at a second cooling speed relatively slow compared to the first cooling speed, and wherein the surface of the reversible thermosensitive recording material on the recording layer side has a ten-point mean roughness (Rz) not less than 1.5 xcexcm, and preferably not greater than 3.5 xcexcm.
Alternatively, the surface of the reversible thermosensitive recording material satisfies the following relationship:
Sm/Rzxe2x89xa6120
wherein Sm represents an average peak-to-peak length of the surface of the recording material, and Rz represents the ten-point mean roughness of the surface of the recording material. The ratio Sm/Rz is preferably not less than 30. Rz and Sm are determined by JIS B0610.
The surface of the recording material preferably has a film strength of grade F or harder, and more preferably grade H or harder when measured by JIS K5400-1990.
The recording material preferably has a structure in which a reversible thermosensitive recording layer, an intermediate layer and a protective layer are overlaid on a substrate in this order. Each of the recording layer, intermediate layer and protective layer includes a resin and preferably a crosslinked resin. In addition, at least one of the recording layer, intermediate layer and protective layer includes a filler, and preferably an inorganic filler.
The recording material may include an information recording portion such as magnetic recording media, IC memories, and optical memories. In addition, the side of the substrate opposite that bearing the recording layer may have an adhesive layer.
The recording material can be used as thermosensitive recording labels, point cards, prepaid cards, consultation tickets, admission tickets, commuter passes, etc., and used for discs, disc cartridges, cassette tapes, cassette cartridges, etc.
In another aspect of the present invention, a reversible thermal image recording and erasing method is provided which includes the steps of heating the recording layer of the reversible thermosensitive recording material of the present invention at a temperature lower than the image forming temperature and not lower than the image erasing temperature such that the recording layer maintains or achieves a non-colored state; and imagewise heating the previously heated recording layer in a non-colored state at a temperature not lower than the image forming temperature and then cooled rapidly to form an image in the recording layer. The heating is preferably performed by a thermal printhead.
In yet another aspect of the present invention, a reversible thermal image erasing (i.e., decoloring) method is provided which includes the step of heating the recording layer of the reversible thermosensitive recording material of the present invention at a temperature lower than the image forming temperature and not lower than the image erasing temperature such that the recording layer achieves a non-colored state. The heating is preferably performed by one of thermal printheads, ceramic heaters, heat rollers, hot stamps, and heat blocks.
In the present application, the term xe2x80x9cerasing (or erasure)xe2x80x9d means the decoloring of recording layers that are entirely colored, or partially colored and partially non-colored.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.